With the recent proliferation of portable appliances and cordless appliances, such as cellular phones and notebook personal computers, there is an increasing demand for batteries that supply electric power to such appliances. Among them, particularly required are secondary batteries that are small and light-weight, have high energy density, and are capable of repeated charge/discharge.
Many of such secondary batteries include an electrode assembly composed of a positive electrode sheet, a negative electrode sheet, and a separator interposed between the positive and negative electrode sheets, which are spirally wound together, and the wound electrode assembly is inserted in a case. Also, the capacities of such batteries are being heightened, so attempts have been made to reduce the diameter of a winding core that is used to form a wound electrode assembly.
However, if the diameter of the winding core is reduced or the packing density of an electrode is increased to heighten the capacity, the electrode may break when wound. Specifically, in the step of winding an electrode, the electrode may break because there is a difference in tensile stress between the inner and outer sides of the wound electrode due to the thickness of the electrode.
In order to solve this problem, various proposals have been made on the step of winding electrode sheets for forming an electrode assembly. For example, Japanese Laid-Open Patent Publication No. Hei 9-283152 proposes passing an electrode plate between a roll with a low surface hardness and a roll with a high surface hardness, to cause micro-cracks in the electrode plate in the direction intersecting with the winding direction. This provides a flexible electrode for use in the wound electrode assembly of alkaline storage batteries.
Also, Japanese Laid-Open Patent Publication No. Hei 11-73952 discloses a technique applied to an electrode plate for use in cylindrical alkaline storage batteries. According to this technique, slit lines are cut in the part of an electrode plate to be positioned outward when wound, at given intervals in the direction intersecting with the winding direction. These slit lines suppress the occurrence of cracks at irregular intervals, thereby suppressing electrode breakage upon winding.
Japanese patent No. 3468847 discloses a technique of forming grid-like grooves in the semidry active material paste applied to a current collector by using an expanded metal, in order to suppress electrode breakage in the winding step.
The use of the technique of No. Hei 11-73952 for suppressing electrode breakage upon winding allows an improvement in the flexibility of electrode plates. However, the method of sandwiching an electrode plate between a roll with a low surface hardness such as a rubber roll and a roll with a high surface hardness allows the electrode plate to be bent only as much as the thickness of the electrode plate at the maximum. Thus, in the winding step, this method does not allow the electrode to be bent more than the curvature of the winding core. Also, although this method can cause micro-cracks in an electrode plate by giving a bend with a small curvature to the electrode plate, such cracks are formed at large intervals. Thus, in the winding step, when a bending force with a large curvature is applied to an electrode plate, a larger compressive stress is exerted on the inner part of the electrode plate. If cracks are formed at large intervals, such cracks cannot scatter the compressive stress sufficiently. As a result, a large stress is concentrated in one crack and the electrode plate therefore breaks upon winding.
In the technique of No. Hei 11-73952, the slit lines in the electrode have a depth of 5 to 10% of the electrode thickness. However, such depth is insufficient for the bending stress to be scattered or released upon winding. Hence, deeper cracks are unevenly produced upon winding, which may result in electrode breakage upon winding.
Further, the application of the technique of JP 3468847 permits formation of grid-like grooves that are deep enough to reach the current collector. However, since this technique uses expanded metal to form grooves, it is difficult to form grid-like grooves at very small intervals. Accordingly, the bending stress exerted on the electrode is not sufficiently scattered and the stress per one crack cannot be reduced, which may result in electrode breakage upon winding.